Wolfies Tweek Pak will let you change your torpedoes without messing with the .cfg files. I've uploaded it to:
http://www.mediafire.com/?s4o2gdegjfp8cr1
If you want to play around manually, here is a guide to adjusting torpedoes in your basic.cfg:
The torpedo type is contained in basic.cfg under [SUBMARINE_AMMO].
As an example, the XXI torpedoes are under [SUBMARINE_AMMO10]. My comments are in red.
ForeTubes=6
Number of forward tubes
AftTubes=0
Number of aft tubes
ForeResIntern=17
Number of internal torpedoes forward
ForeResExtern=0
Number of external torpedoes forward
ForeResExtra=0
Number of extra torpedoes forward
AftResIntern=0
Number of internal aft torpedoes
AftResExtern=0
Number of external aft torpedoes
ShellSlots=10
NbYears=7
Number of years the u-boat is available
Renown=11
Year0=1939
The year you're trying to adjust - each year has a different loadout
ForeTube00=9
The type of torpedo in the first forward tube. Types are below
ForeTube01=9
Second torpedo tube
ForeTube02=9
ForeTube03=3
ForeTube04=3
ForeTube05=3
ForeResIntern00=9
First internal reserve
ForeResIntern01=9
ForeResIntern02=9
ForeResIntern03=3
ForeResIntern04=3
ForeResIntern05=3
ForeResIntern06=3
ForeResIntern07=3
ForeResIntern08=3
ForeResIntern09=3
ForeResIntern010=3
ForeResIntern011=3
ForeResIntern012=3
ForeResIntern013=3
ForeResIntern014=3
ForeResIntern015=3
ForeResIntern016=3
Torpedo types:
[TORPEDO_TYPE0];T I (G7a)
Idx=0 Torpedo type per the list above
Year=1939 Year torpedo is first available
Month=8 Month torpedo is first available
Renown=0 Renown cost
[TORPEDO_TYPE1];T I FaT I (G7a)
Idx=1
Year=1939
Month=8
PrototypeYear=1939 Year prototype torpedo is available
PrototypeMonth=8 Month prototype torpedo is available
Renown=100
[TORPEDO_TYPE2];T II (G7e)
[TORPEDO_TYPE3];T III (G7e)
[TORPEDO_TYPE4];T III FaT II (G7e)
[TORPEDO_TYPE5];T IV (G7es) - Falke
[TORPEDO_TYPE6];T V (G7es) – Zaunkönig I
[TORPEDO_TYPE7];T I LuT I (G7a)
[TORPEDO_TYPE8];T III LuT II (G7e)
[TORPEDO_TYPE9];T XI (G7es) – Zaunkönig II
[TORPEDO_TYPE10]; T VII - Steinbarsch Walter (G7ut)
So... If you want a Type XXI in 1939 with a mixture of Type III and Type XI then change your basic.cfg so it looks like the one above.
If you want to customise my and evan82's XXI 1939 mod further and need some more help just ask - I'm always happy to help out.